Radiation imaging apparatuses are used in a variety of fields ranging from general imaging such as X-ray imaging for acquiring a still image to fluorography for acquiring a moving image. A radiation imaging apparatus uses a sensor panel formed by two-dimensionally arranging, on a substrate, a plurality of pixels, each having a switching element and a conversion element that converts radiation or light from a wavelength converter into electric charges. In particular, a flat panel detector (to be referred to as an “FPD” hereinafter) has received a great deal of attention. In the FPD, a plurality of pixels, each having a conversion element made of an amorphous semiconductor such as amorphous silicon (to be referred to as “a-Si” hereinafter) and a thin-film transistor (to be referred to as a “TFT” hereinafter) made of an amorphous semiconductor are two-dimensionally arranged on an insulating substrate.
PCT(WO) 07-502865 discloses a PIN-type FPD having a plurality of two-dimensionally arranged pixels, each including a PIN photodiode and a TFT. The PIN-type FPD has a stacked structure in which the layer of PIN photodiode is stacked on the layer of TFT on a substrate.
Japanese Patent Laid-Open No. 08-116044 discloses an MIS-type FPD using a sensor panel formed by two-dimensionally arranging a plurality of pixels, each including a TFT and an MIS photosensor made of a-Si. The MIS-type FPD has a planar structure in which the MIS photosensor is made of the same layer as that of TFT on a substrate.
Japanese Patent Laid-Open No. 2004-015002 discloses an MIS-type FPD having a stacked structure in which the layer of MIS photosensor is stacked on the layer of TFT on a substrate.
However, the conventional radiation imaging apparatus can hardly improve the sensitivity of the conversion element. To improve the sensitivity of the conversion element, it is important to increase the output of the conversion element or to reduce noise. Generally, the output can be increased by increasing the opening ratio of the conversion element. When the opening ratio of the conversion element becomes high, its capacitance increases, resulting in an increase in noise. On the other hand, noise reduction can be achieved by decreasing the capacitance of the conversion element. To decrease the capacitance of the conversion element, its opening ratio must be low. If the opening ratio of the conversion element is low, the output decreases.
That is, since the improvement of the output of the conversion element and noise reduction have a trade-off relationship, the sensitivity of the conversion element cannot be expected to be higher.